Epitaxial Electrodeposition of Fe on Au(111): Structure, Nucleation

Article pubs.acs.org/JPCC

Epitaxial Electrodeposition of Fe on Au(111): Structure, Nucleation, and Growth Mechanisms H. F. Jurca,†,‡ A. Damian,†,§ C. Gougaud,†,∥ D. Thiaudière,⊥ R. Cortès,† F. Maroun,† and P. Allongue*,† †

Physique de la Matière Condensée, Ecole Polytechnique, CNRS, 91128 Palaiseau, France Synchrotron SOLEIL, 91192 Gif-sur-Yvette, France

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S Supporting Information *

ABSTRACT: Iron epitaxial electrodeposition on Au(111) substrates is investigated using in situ scanning tunneling microscopy (STM) and ex situ X-ray diffraction (XRD). STM observations show that Fe grows quasi layer-by-layer at sufficiently negative potentials. XRD results indicate that Fe layers thicker than 3 ML are bcc and present the epitaxial relationship Fe(110) [1−10]||Au(111) [11−2]. They also show that the Fe lattice is uniaxially in-plane strained along Fe[1−10] and that the strain is progressively relieved with increasing layer thickness. The growth of Fe on a Ni layer deposited on Au(111) leads to strain free Fe layers with the same epitaxial relationship. The specific shape of Fe of monatomic islands suggests that the first Fe monolayer deposited on Au(111) presents a centered rectangular lattice similar to that of bcc Fe(110) but is stretched along Fe[1−10] by more than 8%. phase transition fcc-Fe(111) → bcc-Fe(110) as a function of the film thickness is generally observed around a critical thickness of 2−3 atomic layers or monolayers (ML). This transition was observed on Ni(111),14−17 Cu(111),18−21 Pt(111),22,23 and Au(111).24−29 Surprisingly, this critical thickness seems to be quasi-independent of the lattice mismatch ε between fcc-Fe(111) and the substrate: ε ∼ +3% if fcc Fe(111) is deposited on Ni(111) but −12% if deposited on Au(111). For films thicker than 3 ML, the exact in plane orientation of the Fe bcc lattice with respect to the underlying fcc lattice has remained a matter of controversy (see ref 15 and references therein) as to whether it follows the NishiyamaWassermann (NW) model or the Kurdjumov-Sachs (KS) model. The main difference between these two models is the bcc Fe(110) lattice in-plane rotation with respect to the fcc (111) substrate lattice (see later atomic models in Figure 7). As mentioned above MBE grown or electrodeposited Fe/ Au(111) layers are exhibiting PMA below a critical iron thickness of about 2−3 ML. It is therefore interesting to compare the nucleation and growth modes, and the structure of Fe/Au(111) obtained by both methods. Scanning tunnelling microscopy (STM) observations revealed a preferential nucleation of compact monatomic islands at the elbows of the reconstructed gold surface in the early stages of MBE

1. INTRODUCTION Ultrathin ferromagnetic films and multilayers are the basic components of data storage and spintronic devices.1 In nowadays devices, layers with out-of-plane magnetization are used because this configuration allows large density data storage stable over time and a writing process which may be localized at the nanometer scale. It has been shown that a perpendicular magnetization may be obtained by sandwiching the magnetic film as Ni, Co, or Fe between two noble metal layers as Au, Pt, and Pd.2,3 The preferential perpendicular magnetization in ultrathin films may be due to an induced elastic strain,4 a specific crystalline structure and orientation favored by the substrate or to specific orbital interactions at the interfaces.2−6 Among ferromagnetic metals, iron has received the widest attention since theoretical works7,8 predicted a complex phase diagram correlating the bulk magnetic properties with the atomic volume and crystal structure. For this reason, iron epitaxial growth was investigated on a variety of single crystal metal substrates with different lattice spacing and crystallographic orientation to vary the lattice mismatch, and the film orientation. Several review articles have been dedicated to the magnetism of iron ultrathin films prepared by physical methods such a molecular beam epitaxy (MBE) or pulse laser deposition (PLD).2,3,8 In the specific case of Fe/Au(111) system, perpendicular magnetization anisotropy (PMA) has been reported with films thinner than 2−3 ML either grown by molecular beam epitaxy (MBE)9,10 or by electrodeposition.11−13 From the structural point of view, when iron is deposited in vacuum on a fcc(111) metal surface, a crystalline structure © XXXX American Chemical Society

Special Issue: Kohei Uosaki Festschrift Received: December 31, 2015 Revised: March 16, 2016

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DOI: 10.1021/acs.jpcc.5b12771 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C growth in the ultrahigh vacuum (UHV)24,25,29and high resolution STM images suggested that the first Fe ML is pseudomorphic with Au(111), that is, is fcc (111) and strained by 12%,25 though a different structure was observed by XRD.27 These nm Fe islands expand isotropically with increasing Fe coverage up to 0.8 ML.24,25 Subsequent Fe growth is then an imperfect layer by layer process and islands thicker than 3 ML present an elongated shape oriented along Au[1−10] directions, which is consistent with NW model (see Figure 7).24,25 The electrochemical growth of Fe ultrathin films (few atomic layers thick) on Au(111) has been also characterized by in situ STM.11−13 These data suggest that Fe electrochemical growth leads to much smoother films on Au(111) than those obtained in the UHV.11−13 However, not much is known about the nucleation stages, the morphology, and structure of Fe layers at low coverage. In this work we investigate the very initial stages of Fe electrodeposition on Au(111) using in situ STM and study the structure of ultrathin films using X-ray diffraction (XRD). Complementary optical reflectivity measurements are also presented to discuss the interaction of Fe with the substrate. The nucleation and growth mechanisms and the thickness dependent film strain are discussed in light of these complementary observations. A comparison with Fe growth on Ni layer deposited on Au(111) is also presented.

with a gold ultrathin layer (ca. 2 nm). The procedure in ref 34 is briefly described. The iron deposition time was adjusted in a trial and error protocol by performing several deposition/ dissolution experiments. The thickness reproducibility of two consecutive deposits in the same conditions is ∼0.2 ML. The final Fe layer was then formed and stabilized at U = −1.3 V. The Fe plating solution was then quickly exchanged with a Fe2+-free electrolyte before adding 0.5 mL of a 10 mM HAuCl4 solution into the cell (ca. 5 mL) while simultaneously shifting the electrode potential to −1.6 V. In these conditions, a very thin (ca. 2 nm) and continuous gold layer is formed within 150 s.34 Au capping procedure may alter the Fe film thickness. For this purpose, we checked it a posteriori by Fe X-ray absorption measurements in fluorescence mode using a four-element silicon drift detector. More specifically, it was a Vortex-ME4 (Hitachi) where the active area of each element is 50 mm2. At each energy step, a region of interest was recorded with sufficient energy resolution to dissociate Kα emission from Fe and Ni elements. An uncapped 40 ML thick Fe layer was used as a reference sample. The thicknesses thus determined are measured with an accuracy ±0.2 ML. They were found to be slightly larger than the one expected from electrochemistry. This difference may be due to some further deposition during the electrolyte exchange prior to depositing the Au capping layer. For this reason, we use the thickness determined by fluorescence for all samples studied by XRD. In situ STM imaging was performed under potentiostatic mode using a home-built microscope described elsewhere.35 The tungsten tips were electrochemically etched and covered with Apiezon wax to reduce the surface area exposed to solution. The MSE reference electrode was connected to the STM cell using a Teflon capillary. The tip potential UT was typically −0.78 V/MSE and the tunneling current 1−5 nA. Images are displayed either in color or in gray scale with heights increasing from dark to bright. All images were acquired on Fe layers grown in the STM cell, either with the STM tip retracted or in tunneling mode, as indicated in the text. X-ray diffraction was conducted either with our custom X-ray bench33,36 or at DiffAbs beamline (synchrotron SOLEIL). The angles and geometry used in this work are defined in Figure S1 (see also ref 33). The Fe film structure and its epitaxial relationship with the Au(111) substrate are derived from Φscans with an incident angle θ, 0.4° < θ < 1°. The origin Φ = 0° corresponds to the X-ray beam oriented parallel to Si[11−2] direction. Please note that the latter is also parallel to the Au[11−2] direction since the Au lattice is aligned with the Si lattice.32

2. EXPERIMENTAL SECTION Before each in situ STM experiment, a freshly prepared Au(111) single crystal (MaTeck GmbH) was used. Experimental details about its preparation are given in refs.30,31 For electrochemical, optical and XRD characterizations, freshly prepared epitaxial 30 ML thick Au(111) buffer layers electrodeposited on n-type Si(111) (10 Ω·cm) were used.32,33 These substrates are flat on a large scale and are particularly suited for ex situ, low angle grazing incident X-ray characterizations (see below). Fe and Ni deposition procedure on Au(111) and Au(111)/Si(111) were identical. Iron and nickel electrodeposition was performed at constant potential in a three-electrode configuration with an Hg/Hg2SO4 (mercury sulfate electrode or MSE) reference electrode and a Pt wire counter electrode. All potentials are quoted against MSE. Solutions were prepared with reagent grade chemicals and ultrapure water. Iron (respectively nickel) was electrodeposited from a 0.5 mM FeSO4 (respectively NiSO4) solution containing 100 mM K2SO4 + 1 mM H2SO4 and 0.1 mM KCl (pH ∼ 3.5). In experiments where iron was deposited on Ni, an epitaxial layer Ni(111)/Au(111) of thickness 1−4 ML thick was first deposited on the Au(111) substrate (see Supporting Information for details about the preparation of the Ni/Au film). For electrochemical characterizations a custom electrochemical flow cell was used (cell volume 2 mL, flow rate ∼ 1 mL/min) with a Au(111)/Si(111) layer as working electrode. The sample reflectivity was measured at normal incidence using a laser diode at 630 nm and a standard Si photodiode for the intensity measurements. The probed sample surface was ∼1 mm2 and the sample area exposed to the electrolyte was 0.5 cm2. The iron growth rate on Au and Ni/Au was determined from deposition/dissolution experiments as explained in Supporting Information. The iron layers to be characterized by X-ray diffraction were grown on Au(111)/Si(111) buffer layers in a different electrochemical cell to ease final capping of the iron layer

3. RESULTS 3.1. Electrochemical Characterizations. In this section we briefly compare the electrochemical response of Au(111)/ Si(111) and Ni/Au(111)/Si(111) electrodes. The two substrates are hereafter called Au and Ni/Au. Figure 1A presents the cyclic voltammograms (CV) recorded at a rate of 50 mV/s with a Au (black line) and Ni(4 ML)/Au (red line) electrodes in the 0.5 mM FeSO4 solution of pH ∼ 3.5. The starting potential is −0.6 V for the Au electrode and −0.96 V for the Ni/Au (to avoid Ni dissolution). Successive potential sweeps give the same CV for both electrodes, which proves in the case of Ni/Au substrate that the Ni layer does not dissolve during cycles of Fe deposition/dissolution within the abovementioned potential range. The stability of the Ni layers is furthermore confirmed by the measured charge upon Ni B


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saturates for U < −1.5 V, where mass transport of Fe2+ species becomes the limiting step (plot c, filled and open black circles). In the case of Ni/Au, the data points (plot d, open red squares) indicate that the Fe deposition at U = −1.5 V is slower than on Au at the same potential (plot c). This is in accordance with the shift of wave (C′2) with respect to wave C2. The mass transport limitation is reached at U = −1.6 V (open red triangles). It is interesting to notice in Figure 1B that tFe − time plots follow a common trend because these are composed of two consecutive linear portions which indicate the presence of two growth regimes: (i) Fe deposition on Au or Ni/Au substrates, and (ii) Fe deposition of the already deposited Fe. The crossover between the two regimes occurs indeed around tFe ∼ 1 ML on both surfaces. A similar behavior was observed during Ni and Co electrodeposition on Au(111).31,37 The second regime is clearly potential dependent, whereas the first one is less sensitive to the potential for potentials more negative than −1.5 V. Finally, it is interesting to note that, on both substrates, Fe deposition rate in the second regime is lower than that in the first regime. We investigated the onset potential of Fe dissolution by examining the electrochemical dissolution current during a positive potential sweep. However, determining the dissolution onset potential using this procedure turned out to be delicate because of the presence of an important baseline originating from the overlapping HER current. In order to overcome this difficulty, we measured the sample optical reflectivity (R) during the same sweep. Since R is only sensitive to the layer thickness, no baseline correction is necessary. Figure 2A

Figure 1. (A) Cyclic voltammograms of Au (black line) and Ni/Au (red line) electrodes in 0.5 mM FeSO4 solution of pH 3.5 (sweep rate 50 mV/s). The initial potential is −0.6 V for the Au electrode and −0.96 V for the Ni/Au to avoid Ni dissolution. (B) Iron thickness vs time for different deposition potentials as indicated in the graph. Black symbols correspond to Fe deposition on Au and red symbols to Fe deposition on Ni/Au. The iron thickness was measured as explained in Figure S3.

dissolution at the end of the experiment (Figure S2, gray line), which yields a Ni thickness close to its expected value. By varying the cathodic limit of the CVs (data not shown) it may be established that the negative current wave C1 (−1.15 V) accounts for the reaction H+ + e− → 1/2H2, also called hydrogen evolution reaction (hereafter HER). This wave is hardly visible on Ni/Au (red line). Peaks C2 (−1.44 V) and A2 (−1.15 V) observed on Au are respectively corresponding to Fe deposition on the negative sweep, and dissolution on the positive sweep following the reaction Fe2+ + 2e− ↔ Fe0. On the Ni/Au electrode the corresponding waves become C′2 (−1.50 V) and A′2 (−1.07 V). Iron films are grown at a constant potential by applying a step from 0.2 V (Au electrode) or −1 V (Ni/Au electrode) to the deposition potential. The Fe thickness tFe cannot be determined from the cathodic charge since Fe deposition takes place in a potential range where HER takes place. Therefore, as shown in previous works,37 tFe is determined from the integration of the anodic charge during Fe stripping which took place immediately after Fe deposition (see Supporting Information, Figure S3). Figure 1B shows the time dependence of tFe for Fe deposition on Au (black symbols) and on Ni/Au (red symbols) and for different deposition potentials. On Au, the Fe deposition rate is practically zero at potentials more positive than −1.35 V where it equals 0.001 ML/s (plot a, filled black squares). Fe deposition rate increases at more negative potentials (U = −1.4 V, plot b, open black squares) and

Figure 2. (A) Potential dependence of the sample reflectivity R upon dissolution of 1 and 2 ML thick Fe layers deposited on Au (filled symbols) and Ni/Au (open symbols). The potential sweep rate is 50 mV for Fe/Au (A) and 10 mV for Fe/Ni/Au (B). The variations of R are the analogue of the one measured in Figure S3 (top panels). These plots are used to estimate the onset potential of Fe dissolution (Udiss; see text). (B) Plot of Udiss as a function of Fe thickness. Note the influence of the substrate below 2 ML. C


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Figure 3. (a) In situ STM image (200 nm × 200 nm) of a Fe/Au(111) film grown at −1.5 V with the STM tip in tunneling conditions. (b) Cross sections along the lines AA′ and BB′ showing that the iron islands consist of several atomic planes. The section BB′ is shifted by 0.4 nm for the sake of clarity. (c) STM image (200 nm × 200 nm) of the deposit imaged in (a) after 10 min at −1.5 V under the STM tip. (d) Image (36 nm × 32 nm) of the deposit in (c) evidencing the corrugation of the iron surface.

thickness is ∼2 MLs. The cross sections shown in Figure 3b indicate that the island height is in the range of 2.6−6.5 Å which corresponds to a local thickness of 1−3 Fe MLs. In addition, the cross sections clearly evidence that the island top surface is particularly smooth. The step height measured between two consecutive Fe planes is about 2 Å, in agreement with expectations for either fcc (111) and bcc (110) iron.25 The island apparent is 2.6 Å with respect to the gold surface (see cross section AA′), a value close but larger than the geometric value of one Fe atomic plane. This apparent inconsistency will be discussed later. Cross section BB′ corresponds to a multilayer island. It is further interesting to notice that the initial Fe islands in Figure 3a present a remarkable morphology with elongated branches along preferential directions separated by 120°. The box in Figure 3a with an enhanced contrast allows determining the orientation of these preferential directions with respect to surface Au dense directions given by blue lines (next to the box in Figure 3A) which are perpendicular to the reconstruction lines. More precise observations of iron monatomic islands are presented below. Figure 3c presents the deposit after ∼10 min of growth. The Fe layer covers most of the Au substrate surface and its average thickness is ∼2.5 ML over the entire image. After initial stages, Fe grows in a quasi-layer by layer fashion: third layer islands indicated by arrows in Figure 3a and c grow exclusively inplane. This behavior is consistent with previous observations.11,12 The close-up image in Figure 3d evidence the absence of any ordered moiré pattern in striking contrast with that observed on Ni and Co/Au(111) films of comparable thicknesses.37 Instead, protrusions with a typical height of 0.3− 0.4 Å are observed. They form a disordered pattern and are sometimes linear.

presents reflectivity curves recorded during the dissolution of Fe films deposited on Au (filled symbols) and Ni/Au (open symbols). All plots start with a plateau which extends until the Fe film starts to dissolve. The drop in reflectivity is indeed corresponding to Fe dissolution current peak (see Figure S3). The second plateau observed at more positive potentials in Figure 2B corresponds to the reflectivity of the bare Au or the Ni/Au substrate. The comparison between the two substrates (open and closed symbols) clearly shows that R drops at more negative potentials for Fe layers deposited on Au. We defined the dissolution onset potential Udiss as the potential for which R deviates by 10% with respect to its value at U < −1.3 V (see Figure 2A). Figure 2B presents thus determined values of Udiss as a function of the Fe thickness for both substrates. The first observation is the increase of Udiss with increasing Fe thickness. A saturation value is reached above ∼3 ML. The second interesting observation is the significant dependence of Udiss on the substrate for tFe < 2 ML, whereas this difference vanishes above 2 ML. It should be noted that the difference Udiss(Au) − Udiss(Ni/Au) is probably underestimated since the sweep rate is 50 mV/s for Fe/Au against 10 mV/s for Fe/Ni/Au. 3.2. In Situ STM Observations of Fe on Au(111). We present below in situ STM observations to characterize the electrochemical nucleation and growth modes of Fe on Au(111). Figure 3 presents STM images recorded during Fe deposition at −1.5 V on Fe/Au(111), that is, with the STM tip in tunneling conditions. The slow scanning direction in Figure 3a is from top to bottom, and the deposition starts close to the top of the image. Slow nucleation of Fe islands is observed and their lateral size and height increase with increasing deposition time. The lateral expansion of the Fe islands is however much faster than their vertical expansion yielding a surface coverage close to 1 at the bottom of the image where the average D


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Figure 4. In situ STM images of submonolayer Fe films deposited at −1.55 V with the STM tip retracted. (a) Image of a 0.4 ML film: Notice the preferential orientations of island edges along the dense direction of the Au(111) surface indicated by the yellow lines. (b) Image of 0.7 ML film; (c) Image showing the dissolution of the deposit in (b) by stepping the potential to −0.6 V during the image scan from bottom to top. Note the nmsized islands left after iron stripping. The deposition time is 0.6 s in (a) and 1.5 s in (b). The deposits are stabilized at −1.3 V for imaging. Image size is 80 nm × 73 nm.

Figure 5. (a) Φ-scan (laboratory X-ray bench) of a 18.8 ML thick Au/Fe/Au(111) structure. Iron was deposited at −1.4 V. Parameters: λ = 1.54 Å (source CuKα1), θ = 0.6°, γ = 39.84°, and δ = 22.15°. The four labeled peaks are consistent with epitaxial relationship Fe(110) [1−10]||Au(111) [11−2] (Figure 7B). The other peaks arise from the 3-fold rotation of the Fe(110) lattice on the Au(111). (b) Narrow Φ-scan around Φ = 0° to highlight the separation angle ΔΦ = 9.6°.

≥1.5 Å with respect to the Fe surface, are assigned to second layer Fe islands. The difference in the Fe morphology and deposition rate in Figure 4 and in Figure 3 may be due to the closer proximity of the STM tip, while imaging Fe deposition in real time in Figure 3. In Figure 4c, the Fe monolayer (imaged in Figure 4b) was dissolved by applying a potential step to−0.6 V during imaging (upward slow tip scanning). Complete dissolution of the film is achieved within a few STM line scans and leaves nm-sized islands (density ∼2−3 × 1012/cm2, height ∼ 2 Å) and also nm holes (ca. 1 Au atomic plane deep) in the gold surface. In the following images (not shown), these islands undergo Ostwald ripening and are thus attributed to Au islands. The holes disappear also progressively. These observations indicate that some intermixing at the Fe/Au interface takes place upon Fe deposition. Similar observations have been reported upon dissolution of epitaxial Co/Au(111) layers.37 3.3. XRD Characterizations. We searched for fcc-Fe(111) and bcc-Fe(110) Bragg reflections on Au-capped Fe layers grown on Au(111)/Si(111) buffer layers. For all deposits discussed below, the diffracted intensity at Bragg conditions for Fe fcc phase was negligibly small compared to that measured at bcc Bragg reflections (the ratio of intensity fcc/bcc was θC (plot b) the Au(−1 0 L) diffracted signal is also arising from the Au(111) substrate. Also interesting, the Fe-related satellite peaks are attenuated by a factor 7 between plots (b) and (c), which stands as a proof that the gold capping layer is homogeneously covering the Fe layer, in agreement with ref 34. The fitting of plot (b) using three Gaussian contributions yielded Φ = −3.02° and 3.40° (ΔΦ = 6.43 ± 0.5°). The red line curve is the contribution from the Au(−1 0 L) rod. For the 3 ML thick iron film deposited on a Ni(1 ML)/Au(111) substrate (plot d), the same fitting procedure gives Fe-Bragg peaks at Φ = −5.15° and 5.25 (ΔΦ = 10.6 ± 0.5°). This indicates that the deposit is a strain free Fe(110) layer. Finally, it is important to note that the full width at half-maximum (fwhm) of Fe/Au(111) peaks is in the range 5.5−6.5° with no clear dependence on tFe (the fwhm value for each Fe thickness is given in Table S1 in the Supporting Information).

4. DISCUSSION In order to understand the epitaxial relation between Fe and Au and Ni/Au substrates, we present in Figure 7A the unit cells of bulk bcc Fe(110) and fcc Fe(111) planes. In Figure 7B,C we show the Nishiyama-Wassermann (NW) and the KurdjumovSachs (KS) models for the epitaxy of bcc Fe(110) on Au(111) and Ni(111) surfaces. We will first consider the NW model: (i) In the case of bcc Fe(110) on Au(111), the Fe bulk lattice

Figure 6. Narrow Φ-scan around Φ = 0° measured at SOLEIL (λ = 1.7712 Å or E = 7 keV). These are the analogue of the Φ-scan in Figure 5b. Plot (a): 5 ML/Au(111) film; Plots (b,c): 3 ML Fe/ Au(111); Plot (d): 3 ML Fe film deposited on Ni(2 ML)/Au(111). All iron layers are deposited at −1.5 V and are capped with 2 nm of gold. The incidence angle θ is given next to plots. Plots (b,d) are decomposed into three Gaussian contributions (dashed lines) with the gray lines related to Fe Bragg peaks and the red line to gold (see text for more explanation).45

1°, as indicated next to each plot (detector azimuth γ = 46.70° and height δ = 25.665°). The above Bragg peaks are expected at Φ = −5.36, 65.16, 174.63, and 245.16° at this energy. For the 5 ML thick film Fe/Au(111) (plot a), we again find two Bragg peaks positioned at Φ = −3.99° and 4.75°, giving ΔΦ = 8.61 ± 0.5°. For the 3 ML Fe/Au(111) film the Φ scans (plots b and c) present an additional contribution centered at Φ = 0°.

Figure 7. (A) Top views of bcc Fe(110) and fcc Fe(111) unit cells. (B) Possible epitaxial relationships for bcc Fe(110) on top of Au(111) according to NW and KS models. (C) Same as (B) for bcc Fe(110) on Ni(111). Fe atoms which should be in Au hollow sites have been inplane shifted to quasi on-top positions for the sake of clarity. F


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of the uniaxial deformation of the bcc Fe(110) lattice ε[1−10] and found a quasi-linear dependence (see Supporting Information, Figure S4). Using this chart plot, relating ΔΦ and ε[1−10], we converted the ΔΦ data points in Figure 8 into ε[1−10] values to plot the experimental thickness dependence of ε[1−10] in Figure 8 (filled symbols, right vertical scale). Clearly there is a rapid uniaxial strain relaxation upon iron growth which may be fitted as α/tFe with α = 25% (solid line). Strain becomes smaller than 2% for tFe > 12 ML, and their relaxation probably occurs by formation of dislocations, which we associate with the faint lines of contrast that are visible on the Fe surface in STM images (Figure 3d). In the case of Fe/Ni/Au(111), we find the same epitaxial relationship, that is, Fe(110) [1−10]||Ni(111) [11−2] (Figure 6) because the Ni(111) lattice is not rotated with respect to the Au lattice37,40 this epitaxial relationship is identical to that of Fe/Au(111). The same epitaxy is found for Fe grown by MBE on Ni(111) in the UHV.15 The absence of any residual strain in Fe layers as thin as 3 ML on Ni/Au(111) suggests that the Fe/ Ni interactions are rather low and do not affect significantly the Fe growth. Similarly to the case of Fe/Au, our results suggest that the bcc Fe(110) lattice configuration with respect to Ni/ Au follows the NW model, in agreement with the literature about Fe MBE on Ni(111).15 4.2. Fe/Au(111) Electrochemical Nucleation and Growth Mechanisms. Given the respective surface energies of Au (1.9 J/m2) and Fe (2.6 J/m2)41 one would expect 3D growth of iron on gold. The first outcome of our STM results is however that the electrochemical nucleation and growth modes of iron on Au(111) is rather two-dimensional at a potential U ≤ −1.5 V. Indeed very good wetting of the Au surface is observed by the first iron atomic layer (Figure 4). Thicker films are also rather smooth and cover very well the gold surface (Figure 3). Iron electrochemical growth on Au(111) appears therefore to be close to a layer by layer in agreement with past work.11 This growth behavior explains the two regimes in the tFe, time plots in Figure 1B: the first regime corresponding to the coverage of the entire substrate by 1 ML of Fe, and the second regime to further Fe/Fe growth. Since plots of tFe versus time of Fe electrodeposition on Ni/Au and on Au have the same typical shape (Figure 1B) with a slope change at around 1 ML, we conclude that iron electrochemical growth on Ni/Au probably follows also a layer by layer growth. This interpretation is consistent with UHV-STM studies which show that the first iron monolayer on Ni(111) wets very well the substrate terraces.16 The electrochemical nucleation and growth modes of Fe on Au(111) at potentials ≤−1.5 V yield films that are smoother than their equivalent obtained by MBE on Au(111).24,25,29 This difference may be assigned to the presence of a monolayer of adsorbed atomic hydrogen (Hads) at the Fe surface in the electrochemical environment. Indeed, recent density functional theory (DFT) calculation have shown that the H-terminated surface is the energetically most stable configuration in the electrochemical environment in the case of hcp Co(0001)42 and fcc Ni(111) (unpublished work). Since the adsorption energy of H is very similar on all iron group metals,43 we expect a similar behavior in the case of Fe. A H-termination is furthermore known to enhance self-surface mobility on Ni in the UHV.44,45 Similar mobility increase is expected in the electrochemical environment and for other iron group metals, which would explain the smoother deposits obtained by electrodeposition.

distance along Fe[1−10] direction (4.05 Å) is 23% smaller than the Au distance along Au[11−2] direction (4.98 Å). On the other hand, there is almost perfect matching (difference < 1%) along the Fe[001] or Au[1−10] direction. (ii) In the case of bcc Fe(110) on Ni(111) the mismatch is ∼+14% along Fe(001) and −6% along Fe[1−10]. Within the KS model an inplane rotation of the Fe(110) lattice by about ±5.2° is inferred allowing to reduce the mismatch along one diagonal of the Fe(110) unit cell, that is, along the Fe[1−11] or Fe[−111] directions. Finally, if one considers the fcc Fe(111) plane, the lattice mismatch is +12% in the case of Au(111) and −3% in the case of Ni(111). We first discuss the film structure above 3 ML, the nucleation and growth modes of Fe/Au(111) and then the monolayer structure. We also discuss the deposition kinetics and the dissolution onset potential of Fe layers on Au and Ni/Au. 4.1. Fe Film Structure with tFe ≥ 3 ML. The XRD data clearly indicate that for Fe films thicker than 3 ML deposited on Au(111), the epitaxial relationship is Fe(110) [1−10]||Au(111) [11−2] (Figure 5). The same epitaxy is found for Fe grown by MBE on the reconstructed Au(111) in the UHV.27,28 Our results suggest that the bcc Fe(110) lattice configuration with respect to Au follows the NW model. Indeed, the ±5.2° lattice rotation in the KS model should induce a splitting of the Fe Bragg peaks in Figures 5 and 6 around each Φ value with a peak separation of 10.4°. Experimentally, the Bragg peak fwhm is ∼6° (see Table S1) for all iron thicknesses. Such a value would have allowed us to observe a peak splitting as in the KS model, which we did not. The peak fwhm of 6° may be due to in plane mosaicity m or to a distribution of lattice strains. Since the fwhm does not depend significantly on the Fe thickness, in contrast with the layer strain (see below), we conclude that there is a moderate mosaicity of the bcc Fe(110) lattice around their preferential position in the NW model, in agreement with the literature about Fe MBE on Au(111).27 The orientation of the elongated branches of the Fe islands on Au(111) (Figure 4a) is also consistent with the NW model. Figure 8 presents the experimental thickness dependence of twin peak separation ΔΦ (open symbols, left vertical scale). This plot evidence that ΔΦ progressively increases and reaches a quasi−plateau above 18 ML where its value becomes close to 10.5°, which is the value expected for a strain free bcc Fe(110) layer. We calculated the theoretical values of ΔΦ as a function

Figure 8. Thickness dependence of ΔΦ (filled circles, left-hand side vertical scale) derived from Φ scans and corresponding variations of uniaxial strain ε[1−10] (open circles, right-hand side vertical scale). Fitting of the ε[1−10] data points with the function α/tFe yields α = 25%, i.e., ε[1−10] ∼ 25% for 1 ML. See text for explanation. All Fe deposits are grown at −1.5 V except the 18.8 ML film, which is deposited at −1.4 V. G


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The Journal of Physical Chemistry C 4.3. Fe/Au(111) Film Structure with tFe < 3 ML. The structure of Fe layers electrodeposited on Au(111) with tFe ≤ 2 ML remains a matter of debate in the absence of atomically resolved STM images. In the UHV, atomically resolved STM images on ML Fe islands indicated pseudomorphism.25 It is therefore very tempting to think that the first electrodeposited iron plane is also pseudomorphic in our case because of the absence of any ordered corrugation. In comparison, Ni(111) ML37 and Co(0001) bilayer37,46 on Au(111), which are quasistrain-free, present a hexagonal periodic corrugation (moiré pattern). If a relaxed fcc Fe(111) monolayer was formed on Au(111) a moiré of period 2.4 nm should be observed. In reality, one only notices disordered nm protrusions in the Fe ML surface (Figure 4B), which means that the Fe ML is not fcc (111). The aforementioned protrusions may be due to the presence of Au in the Fe ML, which origin may be (i) The lifting of the Au surface reconstruction: indeed, as shown by the STM images immediately after Fe dissolution, the Au surface is not reconstructed. The ∼4% excess Au atoms that are ejected from the Au surface upon reconstruction lifting may incorporate into the Fe layer. However, this process does not explain the formation of holes in the Au surface after Fe dissolution (Figure 4C). (ii) Place exchange between Fe and Au similarly to what has been proposed in the case of Co/Au(111):37,47 this process may explain the presence of holes in the Au surface upon Fe dissolution. These holes would be the result of the dissolution of Fe atoms incorporated in the Au surface during Fe electrodeposition. As discussed above, the expression ε[1−10] = α/tFe, with α = 25%, fits reasonably well the data points in Figure 8. It is therefore tempting to extrapolate the data down to 1 ML, which gives a calculated uniaxial strain of 25% for tFe = 1 ML. This value is very close to the uniaxial deformation that is necessary to transform the bcc (110) lattice into a hexagonal lattice that is pseudomorph with Au(111) (see Figure 7). However, the elongated morphology of Fe monatomic island observed in Figure 4A rather suggests an iron lattice with square symmetry, because it is clearly different from the triangular shape observed in UHV.24,25,29,39 One obvious way to break the hexagonal symmetry is assuming that the layer uniaxial strain is slightly less than 23%. Therefore, we propose that the lattice of the first electrodeposited Fe ML is a centered rectangular lattice, similar to bcc (110) lattice, with the largest side slightly smaller than the Au lattice distance along Au[11− 2], which equals 4.98 Å, and the short side, which equals the Au−Au distance (2.88 Å). Unfortunately, one cannot give a precise value for ε[1−10] in absence of atomically resolved STM images. We may definitely state that the first Fe ML uniaxial strain must be in the range 8% ≤ ε[1−10] < 23% because it must be greater than the 8% residual strain measured for tFe = 3 ML. The difference observed between the structure of MBE and electrodeposited iron monolayer islands on Au(111) may again originate from the presence of adsorbed H in the electrochemical environment. As already explained above, the Fe layer is covered by a H overlayer during electrochemical growth. As it is commonly accepted, the presence of adsorbates on a metallic layer likely reduces the interaction energy between the two topmost planes of the layer. In the present case, this will reduce

the interaction between Fe and Au and consequently reduce the driving force for having a pseudomorphic Fe layer. Finally, one could argue that the comparison between the structure of Au-capped Fe/Au layers studied by XRD and the uncapped Fe/Au layers studied by STM is not possible because the upper Au/Fe interface of the capped layers may contribute to the Fe strain. The investigation of Au capped Fe/Ni/Au layers clearly show the absence of any strain for tFe = 3 ML. This clearly demonstrates that the strain which may be induced by the capping layer is negligible. 4.4. Substrate Dependence of Iron Deposition/ Dissolution. As noticed in Figures 1 and 2, the substrate has an impact on the deposition rate and dissolution onset potential Udiss. The H-termination of the Ni surface is a probable origin of the slower Fe growth on Ni/Au as compared to Au because H adsorption energy on Au(111) is very weak.43 Similar influence of adsorbed H on the electrodeposition kinetics has been demonstrated in the case of Ni deposition on PdAu(111) bimetallic surfaces (Ni grows slower on Hterminated Pd).31 Adsorbed H has been also proposed as the origin of the difference in the growth rate between the first ML and that of the following layer in the case of Ni deposition on Au.48 Similar conclusions may be drawn in the case of Fe. A different H adsorption energy on Fe and Ni43 might explain that the growth rate Fe/Fe < Fe(ML)/Ni. Nevertheless, the influence of the substrate is expected to vanish when tFe> 2 ML, which is inconsistent with our observations. Further work is necessary to elucidate this question. The shift of Udiss, in analogy with our previous work,31 is essentially correlated with the binding energy adatom− substrate. We can therefore infer that Fe is less strongly bound to Au than to Ni. Such a dependence is not too surprising in light of a theoretical work that calculated the equilibrium potential of deposition/dissolution for a variety of systems.49 However, these calculations considered exclusively pseudomorphic overlayers, which is clearly not the case here. The origin of the difference in the binding energy may be due to the strain in the Fe layer. This would be consistent with the lower strain in the case of Ni/Au substrate.

5. CONCLUSIONS The growth mode of Fe/Au(111) films is a quasi-layer-by-layer process if a sufficiently negative potential is applied. From the structural viewpoint, the epitaxial relationship bcc Fe(110) [1− 10]||Au(111) [11−2] is found for films thicker than 3 ML. We also found a uniaxial strain in the Fe layer along Fe[1−10], which decays as α (=25%)/tFe. The same epitaxial relationship is found on Ni/Au(111) except that films are strain free. STM observations at submonolayer coverage and complementary electrochemical characterizations suggest that the Fe/Au(111) monolayer presents a centered rectangular lattice which is similar to that of bcc Fe(110) but stretched along Fe[1−10]. The uniaxial strain of the iron monolayer is ≥8%. Fe/Au(111) electrochemical growth could therefore be described, within a unified model, as the growth of bcc Fe(110) with rapid uniaxial strain relaxation along Fe[1−10]. The presence of adsorbed H monolayer on the Fe deposit is thought to play a key role to explain differences with respect to MBE growth. The magnetic properties of Fe films on Au(111) and Ni(111) in relation with the layer structure and morphology will be reported in a separate paper. H


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(11) Gundel, A.; Cagnon, L.; Gomes, C.; Morrone, A.; Schmidt, J.; Allongue, P. In-situ magnetic measurements of electrodeposited ultrathin Co, Ni and Fe/Au(111) layers. Phys. Chem. Chem. Phys. 2001, 3, 3330. (12) Gündel, A.; Devolder, T.; Chappert, C.; Schmidt, J. E.; Cortes, R.; Allongue, P. Electrodeposition of Fe/Au(111) ultrathin layers with perpendicular magnetic anisotropy. Phys. B 2004, 354, 282. (13) Allongue, P.; Maroun, F. Electrodeposited magnetic layers in the ultrathin limit. MRS Bull. 2010, 35, 761. (14) D’Addato, S.; Pasquali, L.; Gazzadi, G. C.; Verucchi, R.; Capelli, R.; Nannarone, S. Growth of Fe ultrathin films on Ni(111): structure and electronic properties. Surf. Sci. 2000, 454−456, 692. (15) Gazzadi, G. C.; Bruno, F.; Capelli, R.; Pasquali, L.; Nannarone, S. Structural transition in Fe ultrathin epitaxial films grown on Ni(111). Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 205417. (16) An, B.; Zhang, L.; Fukuyama, S.; Yokogawa, K. Growth and structural transition of Fe ultrathin films on Ni(111) investigated by LEED and STM. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 085406. (17) Theobald, A.; Fernandez, V.; Schaff, O.; Hofmann, P.; Schindler, K. M.; Fritzsche, V.; Bradshaw, A. M.; Woodruff, D. P. Structure of adsorbed Fe on Ni(111). Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 6768. (18) Ohresser, P.; Shen, J.; Barthel, J.; Zheng, M.; Mohan, C. V.; Klaua, M.; Kirschner, J. Growth, structure, and magnetism of fcc Fe ultrathin films on Cu(111) by pulsed laser deposition. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 3696. (19) Biedermann, A.; Rupp, W.; Schmid, M.; Varga, P. Coexistence of fcc- and bcc-like crystal structures in ultrathin Fe films grown on Cu(111). Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 165418. (20) Passeggi, M. C. G.; Prieto, J. E.; Miranda, R.; Gallego, J. M. A scanning tunnelling microscopy view of the surfactant-assisted growth of iron on Cu(111). Surf. Sci. 2000, 462, 45. (21) Schiechl, H.; Rauchbauer, G.; Biedermann, A.; Schmid, M.; Varga, P. Growth of ultrathin Fe films on Cu(111) by pulsed laser deposition. Surf. Sci. 2005, 594, 120. (22) Cheng, R.; Guslienko, K. Y.; Fradin, F. Y.; Pearson, J. E.; Ding, H. F.; Li, D.; Bader, S. D. Step-decorated Ferromagnetic Fe Nanostripes on Pt(997). Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 014409. (23) Repetto, D.; Lee, T. Y.; Rusponi, S.; Honolka, J.; Kuhnke, K.; Sessi, V.; Starke, U.; Brune, H.; Gambardella, P.; Carbone, C.; Enders, A.; Kern, K. Structure and magnetism of atomically thin Fe layers on flat and vicinal Pt surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, n/a. (24) Voigtländer, B.; Meyer, G.; Amer, N. M. Epitaxial growth of Fe on Au(111): a scanning tunneling microscopy investigation. Surf. Sci. 1991, 255, L529. (25) Stroscio, J. A.; Pierce, D. T.; Dragoset, R. A.; First, P. N. Microscopic aspects ofthe initial growth of metastable fcc-iron on Au(111). J. Vac. Sci. Technol., A 1992, 10, 1981. (26) Allmers, T.; Donath, M. Growth and morphology of thin Fe films on flat and vicinal Au(111): a comparative study. New J. Phys. 2009, 11, 103049. (27) Bulou, H.; Scheurer, F.; Ohresser, P.; Barbier, A.; Stanescu, S.; Quirós, C. Structure of self-organized Fe clusters grown on Au(111) analyzed by grazing incidence x-ray diffraction. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 155413. (28) Dekadjevi, D. T.; Hickey, B. J.; Brown, S.; Hase, T. P. A.; Fulthorpe, B. D.; Tanner, B. K. Structural phase transition of Fe grown on Au(111). Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 054108. (29) Donati, F.; Mairov, A.; Casari, C. S.; Passoni, M.; Li Bassi, A. Nucleation and growth mechanisms of Fe on Au(111) in the submonolayer regime. Surf. Sci. 2012, 606, 702. (30) Damian, A.; Maroun, F.; Allongue, P. Electrochemical growth and dissolution of Ni on bimetallic Pd/Au(111) substrates. Electrochim. Acta 2010, 55, 8087.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b12771. It gives complementary XRD data (fwhm of Bragg peaks) and the correspondence between the separation angle ΔΦ and uniaxial strain ε[1−10] Fe. It details also the Ni/Au electrode preparation, presents the final linear potential sweep corresponding to Ni stripping after multiple Fe deposition/dissolution experiments on a Ni/ Au electrode (Figure 1A) and describes the experimental procedure to determine the Fe thickness plotted in Figure 1B (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses ‡

Instituto Federal do Paraná, Rua Pedro Koppe -n 100, Vila Matilde, 81531980 - Irati, PR, Brasil (H.F.J.). § Eramet Research, 1 avenue Albert Einstein, BP 120, 78193 Trappes, France (A.D.). ∥ Chantereine R&D Centre, 1 rue de Montluçon, BP 40103, 60777 THOUROTTE France (C.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially financed by the Région Ile-de-France in the framework of Cnano IdF (Project Allmagelec) and ANR Seaman. The XRD experiments were conducted at DiffAbs Beamline of the Synchrotron facility SOLEIL (Proposal 20080763).

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Epitaxial Electrodeposition of Fe on Au(111): Structure, Nucleation

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